1. Field of the Invention
This invention generally relates to batteries and particularly to a secondary battery circuit capable of determining a remaining capacity of a secondary battery and to a method of determining the remaining capacity of a secondary battery.
2. Discussion of the Related Art
FIG. 1 shows a battery circuit 25 including a conventional battery condition monitoring circuit 27 that indicates a remaining charge state like that illustrated in Japanese Patent Application Toku Kai Hei 9-312172 entitled xe2x80x9cA Battery Pack, Charging Device, and Charging System and Charting Method.xe2x80x9d The FIG. 1 circuit uses a microcomputer to detect battery voltage and charge-and-discharge current information. Based on measured discharge characteristics, the monitoring circuit calculates the battery""s remaining capacity or stored charge. The microcomputer can transfer information about the battery""s remaining capacity to an external device, such as a portable computer. The external device then is able to display information, such as remaining usable time of the external device.
Battery condition monitoring circuit 27 includes a voltage monitor 19 and a current monitor 18. The battery circuit 25 measures information about the voltage and charge-and-discharge current of secondary batteries 1, 1A, 1B and 1C. The battery circuit 25 predicts the remaining capacity from this information and can then transfer that remaining capacity information to an external device. The voltage monitor 19 and the current monitor 18 transfer the battery condition information to the microcomputer 20.
Voltage detection device 17 detects the level of the secondary battery voltage in comparison to a predetermined battery voltage. A second switch circuit 4 stops the charging of the battery when the battery voltage reaches a predetermined level indicated by the predetermined voltage. Together, the voltage detection device 17 and the second switch circuit 4 prevent a lithium ion secondary battery from overcharging. Switch circuit 3 prevents the lithium ion secondary battery from discharging below a predetermined level indicated by a predetermined voltage. Microcomputer 20 also controls the charging-and-discharging.
Conventional voltage monitoring circuits do not reliably determine the charging state of secondary batteries. For example, many conventional circuits that measure the voltage of secondary batteries fail to account for the internal impedance of the secondary batteries. These conventional circuits may underestimate or overestimate the voltage of a secondary battery depending on when measurements are taken. Because at least some of these conventional circuits detect the secondary voltage by flowing current through a sense resistor, these circuits can waste battery charge and generate heat, possibly adversely affecting the other parts. In addition, conventional battery circuits may be undesirably large because they include many different circuits.
Aspects of the present invention provide a battery circuit capable of accurately detecting a battery remaining capacity, the battery circuit having a secondary battery, a storage capacitor, at least two switches, and two monitor circuits. The first switch connects the secondary battery to the storage capacitor and the second switch, and the second switch connects the storage capacitor to an external load. A secondary battery voltage monitor circuit is coupled to positive and negative terminals of the secondary battery, and a storage capacity voltage monitor circuit is coupled to positive and negative terminals of the storage capacitor. Before the first switch is turned ON and a charge from the secondary battery flows the storage capacitor, the secondary battery voltage monitor circuit measures a terminal voltage of the secondary battery and the storage capacitor voltage monitor circuit measures a terminal voltage of the storage capacitor. After the first switch element is turned ON and then OFF again, the storage capacity voltage monitor measures the terminal voltage of the storage capacitor.
Another aspect of the present invention provides a battery circuit having a capacitor and a monitoring circuit adapted to couple terminals of the capacitor to a battery. The monitoring circuit includes a first switch to selectively couple the capacitor to the battery and a second switch to selectively couple the capacitor to an external load. The battery circuit is charged and discharged in accordance with a voltage measured across the capacitor.
Yet another aspect of the present invention provides a battery circuit having a secondary battery, a capacitor selectively coupled to a load, a monitor circuit coupled to positive and negative terminals of the secondary battery, and at least one switch connecting the secondary battery and the capacitor. When the monitor circuit measures a voltage of the secondary battery, the capacitor supplies current to the load, and the secondary battery is in an open state and does not supply current to the load.
Yet another aspect of the present invention provides a method of measuring a remaining capacity of a secondary battery in a battery circuit and supplying a current to an external load outside the battery circuit. According to this method, first, the secondary battery is selectively connected to a capacitor via a first switch. When the first switch is ON, the secondary battery charges the capacitor. The capacitor is selectively connected to the external load via a second switch. When the second switch is ON, a current flows from the capacitor through the load. The first switch is turned ON and OFF to connect and disconnect the battery and the capacitor. A terminal voltage of the capacitor is measured with a monitor circuit coupled across terminals of the capacitor. The current flows from the battery device to the external load.
Still another aspect of the present invention provides a method of measuring a remaining capacity of a secondary battery in a battery circuit and supplying a current to an external load outside the battery circuit. The method measures a remaining capacity of a secondary battery by charging and discharging the secondary battery in accordance with a voltage measured across a capacitor selectively coupled to terminals of the secondary battery via a switch.